If there is anything we can be sure about the lives of the cave people Og and Thag after they sat around the fire eating roasted mastodon, it is that they didn't take a little purple pill like Nexium to control their heartburn. The condition of burping up acid after eating a meal that we call heartburn, GERD, or gastroesophageal reflux disease, archaeologists tell us, doesn't seem to have been a problem in the paleolithic world.
One of the reasons paleolithic humans did not often have a problem with heartburn was they often did have problems with inadequate food supplies. Gastroesophageal reflux disease and heartburn only occur when acid travels up from the stomach into the throat. In a healthy digestive tract, the esophagus acts like a long pump. It pushes food down and keeps it from coming back up. At the end of the esophagus lies the lower esophageal sphincter. It opens to let food go into the stomach but closes tight to keep stomach acid from traveling back up. And in the stomach, there is a mixture of food and acid that breaks down meals and snacks into the nutrients the body can use for fuel and repair.
Ordinarily, traffic down the esophagus and into the stomach is supposed to be almost entirely one-way. There can be a little “splash” now and then, of course, but not so much that there is pain behind the breastbone, an acid sensation in the throat, or stomach acid coming up into the ears and nose. If there isn't a lot of food in the stomach, or if there's no food in the stomach at all, it isn't hard for the stomach to keep its acidified contents from traveling up from the stomach into the esophagus. If for no reason other than that Og and Thag didn't snag a mastodon every day, paleolithic people probably experienced heartburn very seldom.
But there is another reason paleolithic humans probably almost never had to deal with GERD and heartburn. In the paleolithic world, there weren't any prescription antibiotics. Og and Thag couldn't take Og Junior and Thagette to the doctor to get an antibiotic shot every time the kids got the sniffles. There were no wild elephant feed lots that force fed elephants hay and antibiotics before they were slaughtered for making wild elephant burgers. And as a result, there were fewer aggressive, resistant bacteria just waiting to be eaten so they could make a new home and raise a few trillion babies in the human digestive tract. It turns out that the presence of aggressive bacteria of the “unfriendly” variety is a major reason modern humans get GERD, gastroesophageal reflux disease, and heartburn, and our paleolithic ancestors almost certainly did not. But understanding why bacteria make the difference in GERD requires a little understanding of how the digestive tract works.